what do you mean fairy, there is no pokemon named fairy there is only clefairy. there is also cleffa and clefable, you get clefable with a moonstone and i don't know how to get cleffa. you can only get clefairy at mt. moon.
you go to two island and board the fairy and turn it off as you are in the black.
Use the pokeflute.
Use a gameshark
If there in the way of your path then use the pokeflute.
Yes you can trade leafgreen version Pokemon to another leafgreen version.
use a code
use moon stone
Yes you can.
use thunderstone
Use Cut on the door.
Yes, or a wireless transmitter.
nothing use it as a Pokemon nothing to do with any events